Wideband low frequency impedance tuner

ABSTRACT

Compact electro-mechanical impedance tuners for frequencies as low as 1 MHz use the concept of a cylindrical transmission line with the signal conductor being a spiral wire wound around a metallic cylinder and separated from it by a layer of dielectric material. A conductive wheel is running over the spiral wire and, as the cylinder rotates, the transmission phase between one terminal of the spiral wire and the wheel changes. By connecting a rotating parallel plate variable capacitor to the wheel creates an impedance tuner, capable of generating high reflection factors at very low frequencies at any area of the Smith chart. By cascading two or more such tuners allows tuning at harmonic frequencies at the rate of one frequency per tuner section. The reduction in size comparing with traditional transmission line tuners is of the order of 150:1.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a U.S. Continuation patent application of U.S.application Ser. No. 12/654,463, filed 22 Dec. 2009, the entiredisclosure of which is incorporated herein by reference.

PRIORITY CLAIM

Not Applicable

CROSS-REFERENCE TO RELATED ARTICLES

-   [1] C. Tsironis, U.S. Pat. No. 6,674,293, Adaptable pre-matched    tuner system and method.-   [2] C. Tsironis, U.S. patent application Ser. No. 11,151,419, Low    Frequency Electro-mechanical Impedance Tuner.-   [3] http://www.comet.ch/en/products/vacuum-capacitors/datasheets-   [4] F. Stengel, U.S. Pat. No. 5,771,026, Disguised Broadband Antenna    System for Vehicles.-   [5] C. Tsironis, U.S. patent application Ser. No. 11,643,855, Low    frequency harmonic load pull tuner and method, FIGS. 7 to 10.-   [6] http://www.palstar.com-   [7] AppCad software: http://www.hp.woodshot.com-   [8] C. Tsironis, U.S. Pat. No. 7,135,941, Triple probe automatic    slide screw load pull tuner and method.-   [9] C. Tsironis, U.S. patent application Ser. No. 12,457,187,    Harmonic impedance tuner with four wideband probes and method.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTINGCOMPACT DISC APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

This invention relates to load pull testing of power transistors andnoise testing of low noise transistors using automatic impedance tunersused in order to synthesize reflection factors (or complex impedances)at the input and output of said transistors.

A popular method for testing and characterizing RF components(transistors) for high power or low noise operation is “load pull” and“source pull”. Load pull or source pull are computerized measurementtechniques employing impedance tuners and other test equipment, such assignal sources, test fixtures to house the DUT (device under test,transistor) and input and output power meters (FIG. 1). To measure noisefigure and noise parameters of a low noise device a similar setup isused (FIG. 2), in which the signal source is replaced by a calibratedstandard noise source and the power meter by a sensitive noise figureanalyzer, following a signal amplifying, low-noise amplifier. The tunersin particular are used in both setups, in order to manipulate theelectrical impedance conditions under which the DUT is tested.

A popular family of electro-mechanical tuners, the “slide-screw tuners”,use adjustable metallic probes (slugs) inserted into the transmissionmedia of the tuners [1]. Said transmission media in RF and microwavefrequencies is a slotted straight section of coaxial airline, typicallymanufactured as a parallel-plate structure (FIG. 3). Starting at thecenter of the Smith chart (FIG. 4, (4)) when the RF probe (1) isinserted further (6) into the slot of the slabline (3) the capacitivecoupling with the center conductor (2) increases and so does thereflection factor (7). When the RF probe is moved along the slot andparallel to the axis of the slabline (8) the phase of the reflectionfactor changes until a desired reflection factor (9) is reached. Thisway the whole area of the Smith chart (5) can be covered (FIG. 4).

DESCRIPTION OF PRIOR ART

Full angle coverage of the Smith chart (5) requires the RF probe to movehorizontally at least one half of a wavelength in free space at anygiven frequency. This corresponds to 180° from the test port of thetuner to the RF probe and 180° back, when the probe is at the far endrelative to the test port. The wavelength at any given frequency iscalculated using the simplified formula: Lambda [mm]=300/Frequency[GHz]. At 1 GHz one half of a wavelength corresponds to 15 cm, at 100MHz to 1.5 meters and at 10 MHz to 15 meters. It is obvious thatprecision mechanical structures beyond 1.5 meters are eitherunconceivable or impractical. Therefore the slabline (slotted airline)method has been abandoned. An alternative technique for very lowfrequency tuners has been introduced by Tsironis [2] and uses at leastthree variable rotary capacitors C1 to C3, connected in parallel toground between lengths of low loss coaxial cable L1 to L3 between theinput (10) and output (11) ports, (FIG. 5). This technique allows forvery low frequency tuners, as long as the capacitances are high enough.To determine the value of the capacitance required, in order to generatecertain low impedance, we use the relation: |Z_(min)|=1/(2*π*Frequency[Hz]*Capacitance [Farad]). Typical examples are: For Z_(min)=5Ω at 5MHz, the maximum capacitance needed is: C_(max)=6.4 nF=6,400 pF. Suchtuners need this kind of variable capacitors in order to work. Scalingto lower frequencies and lower impedances is very simple: At 1 MHz themaximum capacitance needed is 31,000 pF and so on.

The only known apparatus [2], allowing user defined impedance tuning inthe low MHz frequency range (1-100 MHz) comprises at least threevariable capacitors, C1, C2, C3, connected to ground and three fixed,low loss transmission lines (L1, L2, L3) interconnecting the floatingpoints of said capacitors (FIG. 5). When the transmission phase betweencapacitors is approximately 60°, at a given frequency (or 120° inreflection), a maximum Smith chart coverage by the reflection factor iscreated. Outside the center frequency, though, Smith chart coveragedeclines (FIG. 6, configuration 3C-3L). So, this solution is notinherently wideband. The same is valid for all alternatives of tuningcircuits using series inductors and parallel variable capacitors [4].

Instantaneous frequency coverage can be improved if more than threeparallel capacitors and associated fixed transmission lines are used.The transmission lines must then be optimized in length for saidfrequency coverage. FIG. 6 shows how the frequency coverage of the 3capacitor-3 line tuner (3C-3L) compares, theoretically, with the hereproposed one capacitor-one phase shifter tuner (1C-1PS). The 3C-3L tunerhas the potential of reaching higher reflection in certain frequencyranges, but lacks wideband coverage. Also, due to the availability ofhigh power variable capacitors (vacuum capacitors [3]) the 3C-3L (ornC-nL) family of tuners offers higher power handling capacity; n being3, 4, 5 or higher. The proposed 1C-1PS tuner is limited in powerhandling due to the sliding (or rolling) contact on the linear phaseshifter (FIG. 8) and we propose alternative solutions for this.

The true equivalent to slabline-based slide screw tuners [1] requires atransmission line (12, 13) and a parallel capacitor (15) connectedbetween an adjustable point (14) on the transmission line and ground(FIG. 7). Though variable rotary capacitors exist that can be used atfrequencies as low as 1 MHz (immersion into a high dielectric constantliquid may be required) and as high as several hundred MHz, without selfresonances, one must always consider the size of the maximum capacitancein relation to self resonance. Self-resonance happens because the leadsto the capacitance C_(max) are in effect series inductances L_(s) (FIG.7) A series resonance occurs when 2π*Freq*L_(s)=1/(2π*Freq*C_(max)), orat Freq=1/(2π*sqrt(L_(s)*C_(max))). The higher the maximum capacitancethe lower will be the self resonance frequency. The major issue is,though, the variable transmission line, needed for such tuners, for suchlow frequencies (sections X, L-X in FIG. 7). Line stretchers (FIG. 16)have been designed, but require still a large number of in-seriessections [5, (FIGS. 7 to 10)] and precision control mechanisms tosucceed. In this invention we propose an alternative solution based on acylindrical structure combined with a variable capacitor, which emulatesaccurately the behavior of a slide screw tuner.

Other than that previous art is poor on the subject of widebandimpedance tuners in the low MHz frequency range.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention and its mode of operation will be more clearly understoodfrom the following detailed description when read with the appendeddrawings in which:

FIG. 1 depicts prior art, a load pull test set-up using passiveelectro-mechanical tuners.

FIG. 2 depicts prior art, a Noise measurement setup.

FIG. 3 depicts prior art, a perspective cross section of slabline basedslide screw tuner.

FIG. 4 depicts prior art, the tuning mechanism of a slide screw tuner.

FIG. 5 depicts prior art, schematics of triple Line-Capacitor lowfrequency impedance tuner.

FIG. 6 depicts partly prior art, a comparison of frequency coverage ofmaximum reflection factor of a tuner with 3 capacitors and 3 fixedcables (3C-3L), a tuner with 5 capacitors and 5 fixed cables (5C-5L) anda tuner with one capacitor and one linear phase shifter (1C-1PS).

FIG. 7 depicts the principle of “one Capacitor—one Phase Shifter”Impedance Tuner.

FIG. 8 depicts a linear phase shifter using a spiraled rotatingtransmission line.

FIG. 9 depicts a cross section of linear phase shifter using a spiraledrotating transmission line.

FIG. 10 depicts the detailed cross section of coaxial adapter transitionto rotating phase shifter.

FIG. 11 depicts a low frequency impedance tuner.

FIG. 12 depicts an automated low frequency impedance tuner

FIG. 13 depicts a automated low frequency impedance tuner with capacitorimmersed in high ∈_(r) dielectric liquid.

FIG. 14 a) depicts prior art, a variable rotary capacitor.

FIG. 14 b) depicts prior art, a variable rotating inductor.

FIG. 15 depicts prior art a): schematics of variable rotating inductor,b) electrical symbol of variable inductor.

FIG. 16 depicts prior art, a) schematics of variable transmission linephase shifter, b) electrical symbol thereof.

FIG. 17 depicts prior art, a cross section of transmission line usingdielectric (wire over ground-plane)

FIG. 18 depicts a) prior art, variable capacitor using concentricconductive cylinders, b) cross section of capacitor.

FIG. 19 depicts linear, high power, phase shifter using a spiraledrotating transmission line and asset of three contact wheelsmechanically joined and electrically connected in parallel.

FIG. 20 depicts a linear, high power phase shifter using a spiraledrotating transmission line and a block of several adjacent andmechanically and electrically interconnected contact wheels.

FIG. 21 depicts a linear, high power phase shifter using a spiraledrotating transmission line and flexible cables to connect to the input,output and variable capacitor, in order to eliminate all not unavoidablesliding contacts.

FIG. 22 depicts a two stage pre-matching or two-frequency harmonictuner.

FIG. 23 depicts a three stage pre-matching or three-frequency harmonictuner.

FIG. 24 depicts a four stage pre-matching or four-frequency harmonictuner.

FIG. 25 depicts a tuner calibration setup.

DETAILED DESCRIPTION OF THE INVENTION

The wideband MHz range impedance tuner comprises, in its simplest form(FIG. 7), one linear phase shifter (92) between test or input port (12)and idle or output port (13); and one variable capacitor (15), which isconnected to said linear phase shifter using some sliding galvaniccontact (14). Changing the position of said sliding contact (14) alongthe transmission line of said linear phase shifter creates a section Xand a section L-X, assuming the total length of said transmission lineto be L. Following the concept of the microwave slide screw impedancetuner, the electrical length of the in-line phase shifter has to be atleast one half a wave-length at the lowest operation frequency. As wehave seen before, the electrical length in air, required at 10 MHz is 15meters and at 1 MHz it is 150 meters. There is no way such dimensionscan be accommodated in a laboratory environment, if a linear approach isadopted.

The alternative approach described in this invention comprises twocomponents: a) the use of some, readily available dielectric material(57), FIG. 17, with a dielectric constant ∈_(r) between 2 and 10. Thisreduces the effective electrical wavelength by a factor of 1/sqrt(∈_(r)). This method is used widely in electrical circuits, in order tosupport the main conductor (58) of transmission lines (56) and to reducesize. The drawback is increased loss, since any dielectric has higherloss than vacuum (or air). The loss/wavelength ratio is roughlyconstant, and depends on the dielectric material and other environmentalconditions such as humidity etc.

In order to reduce the overall length of the linear phase shifter we usea “coil” approach. This technique is available commercially forshort-wave radio-amateurs [6] where big inductors for the MHz frequencyrange are used (FIG. 14 b). Said MHz frequency inductors (coils) areadjustable using a conductive wheel, so they can be combined with fixedcapacitors in order to “tune” to a certain frequency. “Tuning” means, inthis technology, creating a parallel resonance between the variableinductor (FIG. 14 b) and a fixed capacitor, in order to maximize theimpedance and by consequence the voltage detectable across saidimpedance for a given RF current.

In this invention we do not use variable coils, such as shown in FIG. 14b, neither do we use parallel resonances. We use a cylindricaltransmission line structure to create a linear phase shifter and arunning electrical contact (FIGS. 8, 9), in order to reduce the size ofsaid linear phase shifter. This layout bears similarity with thegeometry of the variable coils (FIG. 14 b), but is different in nature,because the linear phase shifter of FIGS. 8 and 9 is not an inductor. Infact the coil of FIG. 14 b does not have a ground plane below the coilwires. Also the coil wires in FIG. 14 b are placed close to each-otherin order to create magnetic coupling and increase the inductance. In thepresent apparatus (FIGS. 8, 9) the coil wires are placed at a distancesuch as to quasi eliminate electro-magnetic field coupling between wires(101). This ensures that the apparatus behaves as a transmission line,with given characteristic impedance Zo, which depends on the wirediameter (58), the thickness (59) and dielectric constant of thedielectric layer (57) between the wire (58) and the ground plane (56) inFIG. 17.

FIG. 8 shows in detail the mechanism of the rotating (24) cylindricaltransmission line; it is made of a metallic cylinder (16) on which alayer of dielectric material (17) has been deposited, either chemicallyor in form of a plastic layer. Typical cheap material is Polyethylene,which is available in sheets of various thicknesses; any otherdielectric material can be used. On top of said dielectric layer ametallic wire (23) (typically aluminum or copper) of certain diameter(typically 3 to 5 mm) is wound in spiral form. The two ends of said wire(93, 94) are bent towards the center of said cylinder and soldered on acentral rotating ring (96, 97) which makes sliding contact with thecenter conductor (18) of a coaxial connector (see also FIG. 10). Themetallic body of said cylinder (which is the ground plane of the wholeassembly) is connected via a number of radial wires (98) with a secondcentral metallic ring (99) which rotates with the main cylinder (16, 17)as well. This second ring (99) makes sliding contact with a fixedmetallic ring (90), which is the outside conductor of a coaxialconnector (see also FIG. 10). The conductors (18) and (90) are thebuilding parts of a coaxial RF connector (FIG. 10). The same assembly ismade on the opposite side of the cylinder (96) and forms the output portof the linear phase shifter. A metallic wheel (made using preferablyaluminum or copper) (20) rolls (22) on the main spiral wire (23) whenthe main cylinder rotates (24); said wheel (20) then slides on theconductor (19) from the far left to the far right side. One end (123) ofsaid conductor (19) is connected with the variable capacitor of thetuner (FIG. 11) thru a sliding contact (40). When the cylinder (16, 17)turns the wheel (20) advances and the amount of spiral wire (23) betweenthe wheel (20) and the capacitor contact (19 a) changes. Thiscorresponds to the adjustable phase X in FIG. 7.

A detailed cross section of the linear phase shifter is shown in FIG. 9;The cylinder (25, 26) rotates around the axis (28) and the metallicwheel (30) rolls on the spiral wire (27) and advances (31) on theconductor (32). The end (109) of said conductor (32) is left open, whilethe other end (108) is connected to the variable capacitor (38) via thesliding contact (40), FIG. 11. The ground plane (26) is connected (102)with the central ring (105) and the conductor wire (27, 100) isconnected with the second ring (104); ring (103) slides on ring (105)and ring (104) slides on the center conductor (106) of the RF connector;both rings (103 and 104) are separated by a dielectric cylinder (107)and lead to the external and internal conductor of the RF connector(FIG. 10). The wires (27) are spaced such (101) that coupling betweeneach spire is small. The electric field is bent (101) towards ground(26) due to the close presence of ground and the dielectric layer (25).

FIG. 11 shows a more detailed cross section of the coaxial RFconnector—cylindrical linear phase shifter transition. Both the groundplane and the main conductor are rotating and are connected permanentlywith two concentric rotating rings. Both said rings make slidinggalvanic contact with the external and internal conductors of the RFconnector. RF power is fed through the coaxial RF connector and thesignal is traversing the spiral wire until it is picked by thetravelling wheel (22 in FIGS. 8 and 30 in FIG. 9) and transferred to thevariable capacitor (38) via another sliding contact (40 in FIG. 11). Thecenter and external conductors of the RF connector are both cylindersand are separated and hold in place thru a nylon cylinder (107), whichalso determines the characteristic impedance of the whole transmissionstructure, which has to be the same over the whole assembly, includingboth RF coaxial connectors and the cylindrical wire-over-groundtransmission line.

The overall low frequency impedance tuner assembly is shown in FIG. 11:The signal enters port (33) and travels on the wire (34) to the output(35). The metallic wheel (113) picks up part of the signal and feeds itto the variable capacitor (38, 39) thru the sliding contact (40). Theaxis (112) of said capacitor rotates and allows a variable amount ofparallel metallic blades (38, 39) to face each other and thus adjust thevalue of the capacitance (FIG. 14 a). The ground plane (36, 37) of saidcapacitor is connected electrically to the external conductor of thecoaxial RF connector (110, 111). The whole assembly is mounted in anenclosure (41), which is then the impedance tuner itself. Typically thisunit is automated by connecting stepper or servo motors to it (FIG. 12);

FIG. 12 shows the overall automated impedance tuner; two remotelycontrolled motors (51, 52) are connected to the rotating phase shiftercylinder (53) and the capacitor axis (47). A simple way to rotate thephase shifter cylinder is to attach a belt (116) on the axis (117) ofthe motor (51) and wound it around the cylinder (53) itself. Motor (52)is rotating the axis (47) of the capacitor (42), of which the groundplane (43, 46) is connected to the ground conductor of the coaxial RFconnector (114, 115). The electrical control wires of both motors (48,49) lead to the system control computer (85, FIG. 25).

Alternatively to the rotating capacitor (42, FIG. 14 a) a coaxialcapacitor can be used (FIG. 18). This later capacitor comprises a set ofconductive cylinders (61, 60) which can be lowered into each other (FIG.18 b) using appropriate gear (63). The ground plane (62) is connected tothe non-moving set of concentric cylinders (61). In this case the axis(50) of the motor (52), FIG. 12, must be modified to allow propercontrol of the capacitor gear (63).

For operation at very low frequencies (below 5-10 MHz) the maximumcapacitance has to be of the order of several tenths of thousands ofPico-Farads, as has been explained before. Typical rotating orinsertable capacitors reach values up to a couple of thousandPico-Farads. To reach higher values of maximum capacitance a simplesolution is to immerse said capacitors into high dielectric liquid. Oilhas a dielectric constant of 2-3 and Alcohol can reach values between 16and 30, depending of the chemical contents. In that case the necessaryvalues of capacitance can easily be reached. FIG. 13 shows a simplearrangement of such a capacitor (55) immersed partially or entirely indielectric liquid (54). The advantages for using this method aretwofold: firstly the spurious inductors Ls (FIG. 7) leading to thecapacitor do not change, as would be the case if we simply addedparallel sections of capacitors in order to increase the capacitance.Secondly, immersion degree can be adjusted to the frequency ofoperation: at lower frequencies we immerse more, at higher frequenciesless or not at all. In all cases the capacitance parts (immersed andnon-immersed) add to the total capacitance.

FIGS. 15 a, b show the schematics of an adjustable inductor (as in FIG.14 b) and the electrical symbol of it. This is shown in order to clarifyand emphasize the structural difference to the adjustable linear phaseshifter of FIG. 16. In FIG. 16 a the schematics of a linear phaseshifter (line stretcher) is shown and in FIG. 16 b the electrical symbolof it. FIG. 17 shows the concept of a transmission line made using the“wire-over-ground” technique, used in this invention. The characteristicimpedance of this transmission line depends on the diameter of the wire(58), the thickness (59) and dielectric constant of the dielectric sheet(57) used. Commercially available software programs allow precisecalculations of said characteristic impedance [7]. Typical values are:For a wire with 3 mm diameter and an dielectric isolator layer with 2.25dielectric constant (Polyethylene) and 50 Ohms characteristic impedancethe thickness of the dielectric layer must be around 3.75 mm. The higherthe dielectric constant the thicker said isolator layer will be.

The electrical length of a linear phase shifter (line stretcher),required for this type of impedance tuner is Lambda/2. At a minimumfrequency of 1 MHz this corresponds to 150 meters in air. If a low costdielectric material, such as Polyethylene with ∈_(r)=2.25 is used, thisbecomes 150/sqrt (2.25)=100 meters. Using a cylindrical structure with adiameter of 30 cm, a 3.75 mm thick layer of the same dielectric materialand a spiral wire with a diameter of 3 mm and allowing for acenter-to-center space between wire spires of 9 mm, in order to minimizemagnetic coupling between spires, it takes a total 106 wire spires andthe total length of the phase shifter becomes approximately 955 mm, orless than 1 meter. This demonstrates the importance of using a spiralline stretcher. Based on this analysis, adaptations of these values todifferent minimum frequencies, diameters of the cylinder, dielectricmaterial and space between wires are possible by simple analogy. Forinstance, if the Polyethylene dielectric is replaced by an Alumina(Ceramic with ∈_(r)=9.8) tube of 15 cm diameter and a wall thickness of20.5 mm, the whole surrounding a grounded metallic cylinder, then thetotal length of said spiral phase shifter will be roughly the same (945mm).

Typical requirements for MHz frequency range impedance tuners includehigh power applications, of the order of 1 or more Kilowatts. In orderto inject high power into a resistor the current to be applied is:Effective Current=sqrt (Power/Resistance) or: I_(eff)=sqrt (P/R); incase of P=1 kW and R=5Ω, the current is I_(eff)=14.14 Amperes. In orderto ensure that the apparatus works reliably, RF currents of this or evenhigher order of magnitude may flow through the rolling contact (30, FIG.9) and shall not degrade the contact too early; it is therefore neededto reduce the current through the contact. This can be done by dividingthe current among several, connected in parallel contact wheels, asshown in FIGS. 19 and 20. The assembly required can be a set of wheelsarranged mechanically in a cascade and connected electrically throughtheir running axes (FIG. 19); it can be seen that wheels (64, 65) and(66) are connected in parallel thru the metallic bars (67, 68, 69) and(70, 71), so that the total current is divided (in this case) by afactor of three. At low frequencies the phase difference between thewheels is irrelevant.

A block of several wheels, also assembled mechanically together (76) andelectrically in parallel (FIG. 20) can also be employed in order toreduce the current thru each wheel. In this case four wheels areconnected as a block (72, 73, 74 and 75) and travel on one or twoparallel axes (118, 119) connected in parallel to the output conductor(77). In this particular case the total RF current is divided by afactor of four. In all cases the current through each rolling contact isdivided by the number of wheels, down to acceptable levels notthreatening the reliability of the apparatus.

An alternative configuration, in which the contacting wheel (120) is nottraveling, is shown in FIG. 21; in this case the cylinder with thespiral phase shifter (83) is moving instead (123). The cylinder (83) isdriven by a threaded axis (78); also the sliding contact between theaxis of the wheel (120) and the rotating (82) axis (121) and movingblades (122) of the capacitor can be replaced by a flexible cable (79).This later technique of using a flexible cable between the axis of thewheel (120) and the axis of the capacitor (121) is applicable to allhitherto configurations of the tuner in this invention and is notrestricted to the configuration of FIG. 21.

In [1], [8] and [9] Tsironis discloses that, when a cascade of two,three or four identical or similar wideband impedance tuners, or tuningsections, is assembled, it is possible to either achieve higherreflection factors, by using the pre-matching tuning mechanism [1], ortune independently at two, three or four typically, but not exclusively,harmonic frequencies (2Fo, 3Fo, 4Fo) of a fundamental frequency Fo, [8,9]; the only condition for this type of operation is that all tuners areable of generating high reflection factors at all said frequencies,being it two, three or four frequencies, simultaneously; if we cascadetwo, three or four identical tuners, as described hitherto in thisinvention, then existing calibration and tuning software algorithms [8,9] can be used, or developed, to do independent tuning at saidfrequencies. FIGS. 22, 23 and 24 show said cascaded tunerconfigurations. Calibrating the cascade of tuners can be done either inassembled form [8, 9], or each tuner can be calibrated separately andthe calibration data cascaded in computer memory.

The possibility of including a second and third independent contactwheel on the same spiral phase shifter wire, in order to create an“integrated” harmonic tuner, as in [8, 9], has been considered as apossibility, but does not offer, in this case, a practical alternativeto the cascade of individual tuners; the reason being that, if allwheels run on the same wire simultaneously, the individual distancebetween wheels will not be adjustable, as is required for independentharmonic tuning. In order for this configuration to work, all wheelsexcept one would have to be lifted from the wire, while the remainingwheel follows the rotation to be placed at a certain phase, then liftedand a second wheel would have to be lowered, be moved into the rightposition, and so on until all wheels are placed properly and all loweredto make contact. This technique is theoretically possible, thoughcumbersome and would require quite a complex mechanical arrangement tomaterialize.

In order to be able to synthesize specific impedances, the tuner must bepre-calibrated [1, 8 and 9]. This is possible using existing methods[1], in which the tuner is connected to a pre-calibrated networkanalyzer and a system computer adjusts the phase shifter and thecapacitor positions such as to cover the reflection factor area on theSmith chart, while simultaneously retrieving scattering parameter datafrom the network analyzer using digital communication, such as GPIB orUSB. FIG. 25 shows the required measurement setup. The tuner (87) isconnected to the network analyzer (84) using low loss coaxial cables(88, 89) while at the same time the control computer (85) controls thetwo tuner motors (86) and reads data through the GPIB interface. Theretrieved data are then saved in tuner calibration files in a formatwhere the motor positions (typically in motor steps) are associated withtwo-port scattering parameters of the tuner and can be used later toreproduce said positions and associated impedances, knowing that thetest port reflection factor is equal to the scattering parameter S₁₁, ifthe output port of the tuner is loaded with the characteristic impedanceZo of the transmission line of the phase shifter.

The present embodiment of this invention can easily be adapted to usealternative materials in order to increase dielectric constant and thesize of the linear phase shifter and different types of variablecapacitors to be connected to the phase shifter. Alternatively a singlecapacitor can be replaced by two or more capacitors connected inparallel, in order to increase the maximum capacitance; this shall notlimit the basic concept and the overall scope of the present inventionin using linear phase shifters and variable shunt capacitors for makinga wideband, low frequency, computer controlled impedance tuner.

What I claim as my invention is:
 1. A low frequency wideband impedancetuner comprising: a cylindrical transmission line, said cylinder beingmetallic, and said cylinder having a longitudinal axis; a dielectriclayer on one surface of said cylinder; at least one wire extendingradially from said cylinder to a first sliding contact; said firstsliding contact being an electrical contact with a second slidingcontact, said second sliding contact being in contact with an outsideconductor of a transmission line; said cylinder, said wire and saidfirst sliding contact being mounted to rotate around said axis; a linearphase shifter; said cylinder being a ground plane when in operation forsaid linear phase shifter, said linear phase shifter being mounted to bein operative electrical communication with said cylinder; a variablecapacitor in operative electrical contact with said linear phase shifterand with said outside conductor of the transmission line; said variablecapacitor and said linear phase shifter thereby being disposed to varyan impedance to a transmission signal.
 2. The tuner of claim 1 whereinsaid electrical contacts with said transmission line are contacts with acoaxial transmission connector.
 3. The tuner of claim 1 wherein saidconnection to said transmission line comprises one of an input or anoutput port and further comprising a third sliding contact and a fourthsliding contact establishing operative electrical connection betweensaid cylindrical transmission line and the other of an input port or anoutput port and said variable capacitor being in operative electricalcontact with the other of said output port or said input port.
 4. Thetuner of claim 1 wherein at least one of said sliding electricalcontacts is annular.
 5. The tuner of claim 1 wherein said dielectric ison an outside surface of said cylindrical transmission line.
 6. Thetuner of claim 1 wherein said linear phase shifter rotates with saidcylinder.
 7. The tuner of claim 1 wherein said linear phase shifter isin operative electrical contact with a center pin of said transmissionline.
 8. An impedance tuner as in claim 1, in which said variablecapacitor is a rotating capacitor with a set of rotating conductiveblades and a set of interdigitally placed fixed conductive blades, thenumber of said blades and their size determining the range ofcapacitance available for tuning purposes.
 9. An impedance tuner as inclaim 1, in which said variable capacitor is a concentric conductivecylinder structure, in which one set of concentric cylinders is fixedand one set is movable and can be lowered interdigitally into the fixedset in order to adjust the capacitance value; said number of concentriccylinders, their height and radius are selected such as to determine therange of capacitance available for tuning.
 10. An impedance tuner as inclaim 1, in which said variable capacitor can be partially or fullyimmersed in dielectric fluid in order to increase and adjust the rangeof capacitance that can be created by said variable capacitor.
 11. Animpedance tuner as in claim 1, in which said variable capacitor isreplaced by a set of two or more such capacitors, connected in parallel,such that a maximum value of capacitance that can be increased.
 12. Animpedance tuners as in claim 1, further comprising a second tunercascaded with said tuner whereby said cascaded tuner can perform atleast one pre-matched tuning, independent two-frequency tuning or twoharmonic frequency impedance tuning.
 13. The tuner of claim 1 whereinsaid linear phase shifter is a spiral wire.
 14. The tuner of claim 13wherein said linear phase shifter is further comprised of a movingcontact disposed to maintain electrical contact with said spiral wireand to translate axially as said cylinder and said spiral wire rotate.15. An impedance tuner as in claim 1, further comprising said linearphase shifter having at least one rolling contact wheel running on awire of a spiral phase shifter.
 16. An impedance tuner as in claim 15,further comprising said linear phase shifter having at least oneadditional rolling contact wheel, assembled mechanically together andinterconnected electrically and running simultaneously on a wire of aspiral phase shifter, such that a current density through each saidrolling contact is reduced.
 17. A low frequency wideband impedance tunercomprising: a cylindrical ground plane; a linear phase shifter inoperative electrical communication with said cylindrical ground plane;said linear phase shifter comprising: a spiral around said cylindricalground plane, said spiral being substantially coaxial with saidcylindrical ground plane; said linear phase shifter rotating with saidcylindrical ground plane; said linear phase shifter being in operativeelectrical contact with a center pin of a linear transmission line via aradial wire; said spiral wire having a plurality of windings around saidcylindrical ground plane, said windings being spaced axially such thatelectromagnetic field coupling between said windings is reduced; saidlinear phase shifter further comprising a travelling wheel, said wheelbeing disposed to translate axially while maintaining electrical contactwith said spiral, as said spiral rotates; said travelling wheel being inoperative electrical contact with an outside conductor of a coaxialtransmission line, said operative electrical contact between saidtravelling wheel and said transmission line being through a variablecapacitor; such that said linear phase shifter and said variablecapacitor are disposed to vary an impedance to a signal through saidtransmission line.
 18. The tuner of claim 17 wherein said operativeelectrical connections with said transmission lines are connections withcoaxial connectors comprising at least one of an input port or an outputport of said tuner.
 19. The tuner of claim 17 further comprising: acylindrical transmission line, said cylinder being metallic, and saidcylinder having a longitudinal axis; a dielectric layer on one surfaceof said cylinder; at least one wire extending radially from saidcylinder to a first sliding contact; said first sliding contact being anelectrical contact with a second sliding contact, said second slidingcontact being in contact with an outside conductor of a transmissionline; said cylinder, said wire and said first sliding contact beingmounted to rotate around said axis; a linear phase shifter; saidcylinder being a ground plane when in operation for said linear phaseshifter, said linear phase shifter being mounted to be in operativeelectrical communication with said cylinder; a variable capacitor inoperative electrical contact with said linear phase shifter and withsaid outside conductor of the transmission line; said variable capacitorand said linear phase shifter thereby being disposed to vary animpedance to a transmission signal.
 20. The tuner of claim 17 furthercomprising a dielectric interposed between said linear phase shifter andcylindrical ground plane.
 21. The tuner of claim 17 wherein a distancebetween one end of said spiral and said travelling wheel defines atleast one of an electrical length and a phase position.
 22. The tuner ofclaim 17 wherein said spiral is a spiral wire.
 23. An impedance tuner asin claim 17, in which said variable capacitor is a rotating capacitorwith a set of rotating conductive blades and a set of interdigitallyplaced fixed conductive blades, the number of said blades and their sizedetermining the range of capacitance available for tuning purposes. 24.An impedance tuner as in claim 17, in which said variable capacitor is aconcentric conductive cylinder structure, in which one set of concentriccylinders is fixed and one set is movable and can be loweredinterdigitally into the fixed set in order to adjust the capacitancevalue; said number of concentric cylinders, their height and radius areselected such as to determine the range of capacitance available fortuning.
 25. An impedance tuner as in claim 17, in which said variablecapacitor can be partially or fully immersed in dielectric fluid inorder to increase and adjust the range of capacitance that can becreated by said variable capacitor.
 26. An impedance tuner as in claim17, further comprising said linear phase shifter having at least oneadditional rolling contact wheel, assembled mechanically together andinterconnected electrically and running simultaneously on a wire of aspiral phase shifter, such that a current density through each saidrolling contact is reduced.
 27. An impedance tuner as in claim 17, inwhich said variable capacitor is replaced by a set of two or more suchcapacitors, connected in parallel, such that a maximum value ofcapacitance that can be increased.
 28. An impedance tuners as in claim17, further comprising a second tuner cascaded with said tuner wherebysaid cascaded tuner can perform at least one pre-matched tuning,independent two-frequency tuning or two harmonic frequency impedancetuning.
 29. A calibration method for an impedance tuner comprising:connecting said tuner to a pre-calibrated network analyzer; measuringscattering parameters for a set of phases of the linear phase shifterand capacitance settings of the variable capacitor, in such a way as tocover a maximum reflection factor area on the Smith chart at any givenfrequency; saving said scattering parameters in tuner calibration filesfor later reproduction (tuning) of said calibrated reflection factors;wherein said tuner comprises; a cylindrical transmission line, saidcylinder being metallic, and said cylinder having a longitudinal axis; alinear phase shifter; said cylinder being a ground plane when inoperation for said linear phase shifter, said linear phase shifter beingmounted to be in operative electrical communication with said cylinder;a variable capacitor in operative electrical contact with said linearphase shifter and with said outside conductor of the transmission line;said variable capacitor and said linear phase shifter thereby beingdisposed to vary an impedance to a transmission signal.